The prior art, notably U.S. Pat. No. 3,845,018 to N. Bilow et al, and U.S. Pat. No. 3,879,349, also to N. Bilow et al, disclose certain acetylene-substituted polyimide oligomers and describe methods for their preparation. The polyimide oligomers are very useful as heat-curable molding powders, coatings on wires or other substances, as adhesives, or as laminating resins. The heat cured product as taught in the '018 patent, column 1, lines 50 et seq., has a very low void content as well as excellent thermal and physical properties. A method of homopolymerizing the acetylene-substituted polyimide oligomers described in the '018 patent can be found in the teachings of the '349 patent referred to above.
The method of preparation of the acetylene-substituted polyimide oligomers is described in the '018 patent and is a multi-step procedure involving the initial reaction of an aromatic tetracarboxylic acid dianhydride such as benzophenone tetracarboxylic acid dianhydride with an aromatic diamine in dimethyl formamide (DMF) at reflux temperature. The product initially formed is an anhydride capped polyamic acid, with some percentage of the amic acid groups being converted to imide groups at the prevailing high temperature.* This intermediate product subsequently is reacted with an amino-arylacetylene in dimethyl formamide at reflux temperature to produce an acetylene-terminated polyamic acid oligomer. Again, by reason of the prevailing high temperature, some of the amic acid groups are converted to imide groups. In accordance with the teachings of the cited prior art, the dimethyl formamide solvent then is removed before the remainder of the amic acid groups of the acetylene-terminated polyamic acid oligomer are converted to the desired imide groups (see col. 4 of the '018 reference, lines 31-33, and the working Examples). The solvent-free oligomer then is diluted with imidization agents such as acetic anhydride or a benzene-cresol mixture. FNT *The formation of imide groups generates water which can react with the anhydride groups to reduce yields and cause formation of chemicals not having the desired structure.
No reason is given for the removal of the DMF from the oligomer before imidization, but, an "imidization" agent as noted is added, presumably to aid in the removal of the water of imidization and thus drive the reaction to completion. When the imidization agent is benzene-cresol, the benzene is used to form an azeotrope with water. The function of the cresol is not specified by Bilow et al in their patents. Since the DMF is removed from the acetylene-capped oligomers, the cresol is believed to function as a solvent for the oligomers to aid in the imidization reaction. The acetic anhydride can serve as both a solvent and water removal agent. In any event, the separate removal of DMF before imidization is an added burden on the process, especially since it is removed in a rotary evaporator requiring considerable energy input and time.
One of the desirable properties of the acetylene-substituted polyimide oligomers is that on curing they have very low void contents, giving rise to excellent thermal and physical properties. It has been found that the ability of the finally cured resin to operate at high temperatures (over 485.degree. F. (252.degree. C.)) for extended periods of time is a direct function of the history of the post-cure temperatures to which the resin is subjected. Thus if the finally cured resin is expected to perform well at temperatures in the 500.degree. to 700.degree. F. (260.degree. to 371.degree. C.) range, then post-curing of the resin also must occur at temperatures of about the same range. It has been observed that when the prior art acetylene-substituted polyimide oligomers are post-cured at temperatures above about 485.degree. F. (252.degree. C.), fracturing or blistering of the resin or laminates prepared therefrom may occur. The reason for failure was found to be the evolution of dimethyl formamide from the resin at temperatures exceeding 485.degree. F. (252.degree. C.), despite the fact that dimethyl formamide has a boiling point of about 302.degree. F. (150.degree. C.), and thus should have been removed by simple distillation or evaporation earlier in the processing sequence. It is theorized that somehow the DMF solvent is coordinating or reacting with components of the acetylene-substituted polyimide oligomers and is not being removed at the expected conditions as taught in the prior art. Extended drying times cannot be employed, as this tends to prematurely cure the oligomers. Even if it were possible to extend the drying time, the dimethyl formamide may well not be removed since heating at 485.degree. F. (252.degree. C.) indicates no DMF was present in a gas stream being analyzed by a pyrolysisgas chromatography-mass spectrometry system.
In the application of Walter Barie, Jr., Ser. No. 782,001, filed on Mar. 28, 1977 now U.S. Pat. No. 4,097,456 (assigned to the assignee of the present application), it was proposed to overcome the above described problems by preparing the acetylene-substituted polyimide oligomers by carrying out all of the reactions, including the final imidization reaction, in N-methyl-2-pyrrolidone (NMP).* The product thus obtained can be cured at temperature above 485.degree. F. (252.degree. C.) without encountering the fracturing and blistering associated with the product prepared in dimethyl formamide. FNT *In this method, the applicant runs all of the reaction steps at about 150.degree. C. or higher. Thus, imide groups are prepared throughout all of the reaction steps with the liberation of water, which causes undesired complications for reasons previously discussed.
A severe limitation of the acetylene-substituted polyimide oligomers is that they have a very low solubility in all known solvents, the maximum solubility being approximately 5 weight % in NMP at 25.degree. C. For this reason it is difficult to impregnate glass fabrics with high loadings of the acetylene-substituted polyimide oligomers which are required to prepare heat-cured laminates of optimum properties.
There is a need in the art for compositions having the desirable properties of the acetylene-substituted polyimide oligomers, but which can be prepared at lower cost and which can be prepared in the form of solutions having a high concentration of resin solids.